An accurate knowledge of spectral properties (for example, a bandwidth) of an optical source such as a laser is important in many scientific and industrial applications. For example, accurate knowledge of the optical source bandwidth is needed to enable control of a minimum feature size or critical dimension (CD) in deep ultraviolet (DUV) optical lithography. The critical dimension is the feature size that needs to be printed on a semiconductor substrate (also referred to as a wafer) and therefore the CD can require tight size control. In optical lithography, the substrate is irradiated by a light beam produced by an optical source. Often, the optical source is a laser source and the light beam is a laser beam.